1. Field of the Invention
The present invention relates to a method for manufacturing transition metal carbides and/or transition metal carbonitrides and the use thereof together with novel transition metal xerogels.
2. Description of the Prior Art
Transition metal carbides are because of their great hardness of major importance for the hard metal industry. They are used in particular for the manufacture of cutting tools. Hardness and wear resistance increase, moreover, with decreasing grain size of the carbides. The manufacture of particularly fine-particle carbides, in particular those with grain sizes of less than 1 .mu.m, is therefore of great interest.
The manufacture of transition metal carbides from their oxides by solid-phase carburization with graphite at temperatures of between 1400.degree. and 2000.degree. C. is known. Sub-.mu. carbides are obtainable in this manner only by laborious grindings of the products or by the use of expensive, extremely finely-divided educts.
WC powders with mean particle sizes of less than about 0.5 .mu.m cannot be prepared in this manner, and some novel methods of preparation have therefore been developed in recent times.
In DE-A 4 214 725 a gas-phase process is described with which extremely finely-divided particles for non-oxide ceramics can be produced. This process is highly complicated in equipment terms, however, as is also the process described in WO 91/07244 for the manufacture of WC/Co composites by the spray drying of co-precipitated W/Co salts and subsequent reduction to the metal powders with downstream carburization to the carbidic WC-Co mixed phase.
In U.S. Pat. No. 5,372,797 the carburization of tungsten trioxide in a reactive gas atmosphere consisting of hydrogen and methane is described.
In EP-A 0 239 301 the manufacture of carbide precursors of the 4th to 6th sub-groups by transesterification of the metal alkoxides with polyols and subsequent precursor pyrolysis to the desired carbides is described.
In JP-A 56-155 013 a similar method is described, with the difference that the metals can be used not only as alkoxides, but also in the form of their halides and oxohalides.
The known solutions have the following disadvantages: The conventional method of the solid-phase carburization of metals or metal oxides with graphite powder requires as a rule temperatures of more than 1400.degree. C. and yields products with mean particle sizes of more than 1 .mu.m, which have to be worked up with a correspondingly high amount of grinding and sorting. In addition, extensive grindings in general impair the powder properties.
The carburization in a reactive gas atmosphere yields as a rule sub-stoichiometric products.
Precursors from alkoxides are too expensive for hard metal powders, since the alkoxides required are not available commercially in large amounts and their manufacture is complicated. Precursors containing halides are not very suitable on environmental grounds.
The object of the invention was therefore to provide a simple process which is suited to manufacturing sub-s carbides and carbonitrides of the 5th and 6th sub-groups with mean particle sizes of less than 0.5 .mu.m on a large scale and at a modest price from commercially available, low-cost educts and thereby to meet the increasing requirements as to their finely divided character.